Field of the Invention
The present invention relates to a golf ball having a cover made of a thermoplastic material and to a method of manufacturing such a golf ball. More specifically, the invention relates to a golf ball which, by the application of a specific treatment to the surface of the ball cover that has been molded from a thermoplastic material, is endowed with an excellent scuff resistance and spin properties, and relates also to a method of manufacturing the golf ball.
Prior Art
The use of polyurethane materials as golf ball cover-forming materials in recent years is noteworthy. Polyurethane materials, from the standpoint of the molding method used to obtain moldings therefrom, are broadly divided into thermoset polyurethane materials and thermoplastic polyurethane materials. Moldings of the former (thermoset polyurethane materials) can be obtained by mixing under applied heat a urethane prepolymer having isocyanate end groups with a polyol or polyamine curing agent in the form of a liquid starting material, then pouring the mixture directly into a mold and heating to effect a urethane curing reaction.
Numerous golf balls that use such thermoset polyurethane materials have been disclosed in the art, including those described in U.S. Pat. Nos. 5,334,673, 6,117,024 and 6,190,268. Methods of molding thermoset polyurethane materials are disclosed in, for example, U.S. Pat. Nos. 5,006,297, 5,733,428, 5,888437, 5,897,884 and 5,947,843.
Moldings of thermoset polyurethane materials have no plasticity when heated, and so the starting materials and molded articles made therewith cannot be recycled. Moreover, in the production of such moldings, the thermosetting step and the cooling step take a long time, in addition to which the starting materials have a high reactivity when heated and are thus unstable, making the molding time very difficult to control. Hence, the productivity of thermoset polyurethane materials when used to make special moldings such as golf ball covers (moldings which encase a core material) is regarded as inefficient.
By contrast, moldings of thermoplastic polyurethane materials are not obtained by the direct reaction of starting materials; instead, a linear polyurethane material synthesized by employing starting materials and a manufacturing process which differ somewhat from those for the foregoing thermoset polyurethane materials are used in molding. Such polyurethane materials are thermoplastic; thermoplasticized polyurethane materials have the quality of hardening when cooled. Accordingly, it is possible to mold such polyurethane materials using an injection molding machine. The injection molding of thermoplastic polyurethane materials has a very short molding time compared with the molding time for thermoset polyurethane materials and moreover is suitable for precision molding, thus making it ideal as a process for molding golf ball covers. Furthermore, thermoplastic polyurethane materials are recyclable, and thus friendly to the global environment. Golf balls which use thermoplastic polyurethane materials are disclosed in, for example, U.S. Pat. Nos. 3,395,109, 4,248,432 and 4,442,282.
However, golf ball covers made from conventional thermoplastic polyurethane materials leave something to be desired with regard to all of the following: the feel of the ball at impact, controllability, rebound, and scuff resistance on shots with an iron.
To address this problem, JP-A 9-271538 describes a golf ball cover made from a thermoplastic polyurethane material having a high resilience. Yet, even this golf ball cover falls short in terms of scuff resistance on shots with an iron.
JP-A 11-178949 describes a golf ball cover which is composed primarily of the product obtained by reacting a thermoplastic polyurethane material with an isocyanate compound and has a relatively good scuff resistance on shots with an iron. In this cover, an isocyanate compound which is a blocked diisocyanate or an isocyanate dimer is added as an additive to a thermoplastic polyurethane material. This method of addition, by adding the isocyanate compound during melt mixture under applied heat using an extruder or during injection molding, is designed to bring about a reaction during molding.
However, in molding the cover in JP-A 11-178949, the isocyanate compound is prone to deactivation by moisture and thus difficult to handle, making it challenging to obtain a stable reaction product. Also, blocked isocyanates, which are strongly hygroscopic, emit a powerful blocking agent odor when they dissociate under the effect of heat, and thus are unsuitable for molding covers. In addition, when the isocyanate compound is in the form of a powder or solution, the amount of addition to the thermoplastic polyurethane material is difficult to control, which in turn has made it difficult to control the physical properties of the cover. Moreover, owing to the difference between the melting points of the thermoplastic polyurethane material and the isocyanate compound and the difference in their melt viscosities, slippage arises within the molding machine, which sometimes makes thorough mixture impossible to achieve. In the foregoing published art, owing to the above causes, control of the influence by moisture and of the additive loadings in the cover material is inadequate. As a result, it has not been possible to obtain a golf ball cover that is entirely satisfactory in terms of improving the scuff resistance.
Also, the preferred thermoplastic polyurethane material mentioned in JP-A 11-178949 above is based on an aliphatic isocyanate. However, because this thermoplastic polyurethane material has a very large reactivity with isocyanates, making the reaction difficult to control, there have been a number of problems. For example, gelation tends to arise prior to use of the material in injection molding, making it impossible to ensure sufficient plasticity; gelation sometimes occurs during molding; and recycled resin cannot be reclaimed on account of gelation. These problems have made it difficult to put the art described in the above publication to practical use.
JP-B 58-2063 (and the corresponding U.S. Pat. No. 4,347,338) disclose a method of manufacturing thermoset polyurethane molded articles by intimately mixing a compound having two or more isocyanate groups with a thermoplastic resin that does not react with isocyanate groups, blending the resulting mixture with a thermoplastic polyurethane material, then feeding the blend to a molding machine and molding. However, the object of this published art is only to improve solvent resistance and resistance to repeated wear; these publications make no mention of the use of this molding material as a cover material for golf balls. It is desired that golf ball cover materials be materials which satisfy the following properties required of golf balls: rebound, distance, spin properties, controllability, feel at impact, scuff resistance, cut resistance and resistance to discoloration.
JP-A 2002-336378 describes a golf ball obtained using a cover material composed of a thermoplastic polyurethane material and an isocyanate mixture. The cover material is a thermoplastic polyurethane material that is recyclable and moreover has a high resilience and an excellent scuff resistance. This cover material makes it possible both to achieve the good productivity of a thermoplastic polyurethane and to exhibit physical properties comparable with those of a thermoset polyurethane; at the same time, due to the plasticizing effect by the isocyanate compound, this approach enhances the flow properties of the thermoplastic polyurethane material and is thus able to improve productivity. Although this art is outstanding in the above respects, because burn contaminants arise due to direct charging of the isocyanate mixture into the molding machine and there is some variability in the compounding ratio owing to the use of dry blending, the uniformity is poor, giving rise to molding instability. At the same time, the compositional ratio within the isocyanate mixture between the isocyanate compound and the thermoplastic resin that is substantially non-reactive with isocyanate has already been set, and so one has less freedom of choice in the amounts and types of isocyanate compound and thermoplastic resin to be added.
JP-A 2002-336380 describes a golf ball which uses, as the cover stock, a material obtained by compounding a thermoplastic polyurethane material that contains, as a polymeric polyol, a polyether polyol having an average molecular weight of at least 1500 and has a rebound resilience of at least 40% with a specific isocyanate mixture. However, as in the case of the foregoing patent publication, there are a number of undesirable effects, such as the generation of burn contaminants due to charging of the cover stock into the molding machine, molding instability, and also limitations on selection of the loading and type of isocyanate compound added.
JP 5212599 describes a golf ball which has a high rebound and an excellent spin performance and scuff resistance, and also has a high cover material flowability and a high productivity. However, the thermoplastic polyurethane material used in this art is a special resin mixture, and there are challenges regarding the production, supply and cost of such a resin mixture. Moreover, because this art entails charging an injection molding machine and ancillary equipment with a material in which isocyanate groups remain in an unreacted state, due to deposition of the isocyanate ingredient, undesirable effects such as seizing and solidification arise that can cause an increase in the percent defective.
JP 3494441 discloses art where, in a golf ball having a cover made of thermoplastic resin, the surface layer of the cover is subjected to modification treatment, thereby giving a cover having excellent properties. However, although this is an excellent approach that enables the properties to be modified after the cover has been molded of a thermoplastic material of excellent moldability, there remains room for improvement in the degree to which the scuff resistance is enhanced and in the golf ball properties following modification. Particularly in cases where 4,4′-diphenylmethane diisocyanate (MDI) has been selected as the polyisocyanate compound, the MDI must be held at the solidifying point (about 39° C.) or higher in order to place it in a molten liquid state suitable for treatment. In the case of MDI, production of the dimer also requires care; that is, dimer production generally speeds up at a higher temperature, and so close temperature control is required to ensure stable productivity, which can be troublesome. Moreover, in cases where an ionomer material is used in part of the golf ball, there is also a possibility that treatment at a high temperature will give rise to deformation or a change in properties.
JP 4051374 describes a method of manufacturing golf balls produced with a thermoplastic polyurethane, polyurea or mixed material thereof having a high melt index. More specifically, this method includes the step of obtaining a base thermoplastic polyurethane, polyurea or mixed material thereof; the step of raising the melt index of this base resin material; the step of molding the thermoplastic polyurethane material having an increased melt index in a mold; and the subsequent step of treating the molded material with a secondary curing agent such as an isocyanate solution. This art is outstanding in that, by crosslinking the cover material using an isocyanate ingredient, resistance to damage can be imparted while maintaining the other desirable characteristics of the cover, such as the softness and feel. However, in cases where the secondary curing agent includes an organic solvent capable of causing the cover material to swell, depending on the treatment conditions, this may cause deformations on the order of several microns in the dimple shape, which may adversely impact the flight performance, and the appearance of the golf ball may worsen due to solvent infiltration to the cover material interface that arises during molding.
It is therefore an object of this invention to provide a golf ball which, compared with conventional golf balls, has an even better scuff resistance and spin performance, and moreover has an excellent manufacturability. A further object of the invention is to provide a method of manufacturing such golf balls.